Reinhardt, V. 1991. Group formation of previously single-caged adult rhesus macaques for the purpose of environmental enrichment. Journal of Experimental Animal Science 34, 110-115.

An attempt was made to form two groups of 6 previously single-caged adult female and 6 previously single-caged adult male rhesus macaques. The study was based on the premise that simultaneous introduction of familiar animals reduces the risks involved during group formation. Future group members were therefore given ample opportunity to physically interact with each other on a one-to-one basis and were considered ready.for group formation only when they had demonstrated compatibility and clear-cut dominance-subordination relationships. Both group formation attempts were failures. Aggressive incompatibility was heralded by certain subjects challenging other partners to whom they had originally been subordinate. Aggressive harassment was intensive and persistent. Victims showed no resistance, except for fear-grinning and crouching In submission. They did so to no avail. Both groups were split again within the first hour of introduction to avoid fatal consequences. It was concluded, that unlike pair-housing, group-housing of previously single caged adult rhesus macaques is associated with considerable risks which cannot be overcome by systematically familiarizing all group members before the animals are introduced as a group. There is no guarantee that well established dyadic relationships do not break down within the more complex social structure of the group, thereby triggering aggressive conflicts that put the animals' safety into undue jeopardy.